[PPT] - Forest Fuels and Fire Forest Fuels and Fire Behavior Behavior PowerPoint Presentation

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Forest Fuels and Fire Forest Fuels and Fire Behavior Behavior

Rick Everett Rick Everett ESPM 134 ESPM 134 Spring 2008 Spring 2008

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Where we’re going: Where we’re going:

Fuels: What they are

Fuels: What they are

Fuel Classes

Fuel Classes

Fuels & Fire Hazards

Fuels & Fire Hazards

Fire Behavior

Fire Behavior

Fuels, Topography, Weather

Flame & Flame front Characteristics

Spread Characteristics

Fire Behavior Measurements

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Fuels Classification Fuels Classification

Natural fuels

From natural forest growth and death.

Includes all leaf and branch materials

Includes all duff, and litter

Activity fuels

Human

Human-

caused

caused

Logging

Thinning

Herbicide Use

Recreation

Dead, downed & remnant

All dead & down materials on the forest floor

Can be activity or natural

Either sound or rotten

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Timelag Theory Timelag Theory

Timelag Principle: wetting and drying of fuels

Timelag Principle: wetting and drying of fuels

Timelag: the amount of time required for a fuel to

Timelag: the amount of time required for a fuel to reach approximately 63% of the difference between reach approximately 63% of the difference between initial moisture content and equilibrium moisture initial moisture content and equilibrium moisture q content content

The 63% is a step function solution: 1

The 63% is a step function solution: 1-

1/e = 0.63, where

1/e = 0.63, where ‘e’ is the base of the natural log. ‘e’ is the base of the natural log.

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Timelag Classification Timelag Classification

The amount of fuel exposed to the atmosphere

The amount of fuel exposed to the atmosphere is related to fuel size is related to fuel size

Small diameter fuels have a large surface to area ratio

Change rapidly with changes in temperature and humidity

Big diameter fuels have a small surface to volume

Big diameter fuels have a small surface to volume ratio ratio

Change slowly with changes in temperature and humidity

Timelag increases with increasing Fuel diameter

Timelag increases with increasing Fuel diameter

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Fuel Classes Fuel Classes

Classified by how quickly they can respond to changes in atmospheric

Classified by how quickly they can respond to changes in atmospheric conditions conditions

1-

hour timelag fuels: 0.0 to 0.25”

hour timelag fuels: 0.0 to 0.25”

10

10-

hour timelag fuels: 0.25 to 1.0”

hour timelag fuels: 0.25 to 1.0”

100

100-

hour timelag fuels: 1.0 to 3.0”

hour timelag fuels: 1.0 to 3.0”

1000

1000-

hour timelag fuels: 3.0 to 8.0”

hour timelag fuels: 3.0 to 8.0”

Sampled using a line intercept transects (the “Brown Transect”

Sampled using a line intercept transects (the “Brown Transect”

Sampled using a line intercept transects (the Brown Transect

Sampled using a line intercept transects (the Brown Transect

# of fuels intersecting 30m line can be converted to tons/unit area

Usually thought of as a “% fuel moisture”:

Usually thought of as a “% fuel moisture”:

If a 1 hour fuel stick is carrying only 10% of what is its tested maxima for

If a 1 hour fuel stick is carrying only 10% of what is its tested maxima for moisture, then the % fuel moisture is 10% moisture, then the % fuel moisture is 10%

Easy to test, 0

Easy to test, 0-

100% scale

100% scale

Live: 0

Live: 0 – – 200%; 200%;

Critical levels < 100 %

Critical levels < 100 % -

half of the normal relative moisture

half of the normal relative moisture

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Fuels: RAWS Data availability

real time
set for models

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Fuels and Fire Hazards Fuels and Fire Hazards

Wildland Fuels are divided into 4 classes:

Ground

Duff

Duff – – moist upper A horizon, decomposing litter moist upper A horizon, decomposing litter

Not a big player in fire behavior

Surface

All dead downed woody stuff, litter, grasses, herbs, shrubs

dd dd

Ladder

Fuels that provide vertical continuity from Surface to Crown

Crown

Overstory biomass

Overstory biomass Reductions of these fuels limit fire intensities

Reductions of these fuels limit fire intensities

Fuel treatments!

Surface

Surface – – Ladder Ladder -

Crown

Crown

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Fire Behavior Fire Behavior

3 Interacting classes of influencing variables:

3 Interacting classes of influencing variables:

Fuels

Topography

We ther

Weather

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Fuels Fuels

Already got it…

Already got it…

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Topography Topography

Slope

Fires on slopes can

Increase flame length,

Preheat upslope fuels

These can increase intensity and rate

These can increase intensity and rate-

of
f-spread

spread y p p Terrain

Terrain

Topography can influence:

Slope (!)

Diurnal air movement

Aspect: fuels temperatures

Physiography:

Chimneys, saddles, box canyons

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Chimney, slopes: NOT a good place to continue standing

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Weather Weather

Temperature

Relative Humidity

Precipitation

Atmospheric stability

Vertical stability (inversions), advection

Winds & Windspeed

General

General – – broad scale pressure gradients broad scale pressure gradients

Foehn Winds: Santa Anas, Diablos

Local

Sea breezes, land breezes, slope winds

Fire Whorls

Critical: weather can change rapidly, changing fire behavior

Critical: weather can change rapidly, changing fire behavior rapidly rapidly

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Weather Weather

Whorls: 10 to 1000’s m2, increase intensity eight-fold

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Weather Weather

Thunderstorms

Thunderstorms

Violent

Violent up/downdrafts up/downdrafts

Lightning

Lightning S i i d S i i d

Sometimes associated

Sometimes associated with cold fronts with cold fronts

Downdrafts often lead

Downdrafts often lead to violent, lateral front to violent, lateral front winds winds

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Fire Movement Fire Movement

4 Processes: Convection, Conduction, Radiation,

4 Processes: Convection, Conduction, Radiation, Mass Transfer Mass Transfer

Convection:

Conduction is the transfer of thermal energy (Q) from a

Conduction is the transfer of thermal energy (Q) from a region of higher temperature (A) to a region of lower region of higher temperature (A) to a region of lower temperature via direct molecular communication between temperature via direct molecular communication between mediums in direct physical contact without a flow of the mediums in direct physical contact without a flow of the material medium material medium

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Convection Convection

a mechanism of heat transfer occurring because

a mechanism of heat transfer occurring because

f bulk motion (observable movement) of fluids
f bulk motion (observable movement) of fluids

Air is a fluid…

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Radiation Radiation

Thermal radiation

Thermal radiation is electromagnetic is electromagnetic radiation emitted radiation emitted from the surface from the surface

f an object
f an object

which is due to which is due to the object's the object's temperature. temperature.

Infrared radiation

Colorado DNR

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Mass Transfer Mass Transfer

Phrase commonly used for physical processes

Phrase commonly used for physical processes that involve molecular and convective transport that involve molecular and convective transport

f atoms and molecules within physical systems:
f atoms and molecules within physical systems:

Moving stuff (usually gaseous) by convection

Molecules

Sparks (glowing embers)

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Flame Characteristics Flame Characteristics

Flame Height

Vertical distance flame extends

Vertical distance flame extends beyond fuels bed beyond fuels bed

Tough to measure…flames pretty

Tough to measure…flames pretty ephemeral ephemeral Flame Length

Flame Length

Distance from base of flame zone

Distance from base of flame zone to tip of continuous flames to tip of continuous flames to tip of continuous flames to tip of continuous flames

Used for intensity

Highly variable: fluctuates due to

Highly variable: fluctuates due to wind, fire behavior, fuels wind, fire behavior, fuels

Flame Angle

θ : angle between ground and

: angle between ground and average flame average flame

Influenced by winds, convection

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Flame Characteristics Flame Characteristics

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Fire Spread Fire Spread

Spread from a point ignition

Spread from a point ignition

Roughly elliptical shape

Aligns central axis with

Prevailing wind

Slope

Both

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Fire Spread Fire Spread

Three main parts, used in perimeter and

Three main parts, used in perimeter and behavior descriptions: behavior descriptions:

Head (heading)

Flames blown towards unburned fuels

Flank (flanking)

Alternatively backing and heading

Back (backing)

Moves into the wind, or against a slope

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Wildfire Spread Wildfire Spread

Wave (like waves in a pond)

Low to Moderate Intensities

Spotting

High intensity

Easily places fire 1.5 miles away

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Fire Behavior Measurements Fire Behavior Measurements

Two Equations can be employed:

Fireline intensity (I):

I = 259L

I = 259L217

217 I is given in kW/m and

I is given in kW/m and

L is flame length in meters (m)

I L = 259

217 .

L is flame length in meters (m).

I = h

I = hωr

I is still kW/m

ω is fuel loading in kg/m

is fuel loading in kg/m2

r is the rate of spread in m/sec

This equation tends to overestimate; need to know difference between

This equation tends to overestimate; need to know difference between flaming combustion and smoldering combustion flaming combustion and smoldering combustion

Old standard (still used in structural stuff): BTU/foot/sec